1. Field of the Invention
The present invention relates generally to a controlled oscillator, and relates more particularly to a multiple range oscillator with selectable inputs for range selection.
2. Description of the Related Art
Oscillators, and especially on chip oscillators, are used in numerous applications involving integrated circuit products, for example. In particular, saw tooth oscillators are used in many applications and are traditionally implemented by providing a current source that charges a capacitor. The capacitor is charged and discharged to produce an oscillating output. Voltage controlled oscillators also provide an oscillator output based on a control voltage to determine the frequency of the output saw tooth waveform.
It has often been the case that a high precision current source is used to charge the capacitor to establish a precision frequency. The precision current source is typically controlled with a precision resistor. Because of the precision required, the high precision resistor is typically provided as an external resistor to an integrated circuit that includes the current source. It would be desirable to obtain a completely integrated current source to obtain a precision oscillator output.
A VCO likewise uses a high precision resistor to obtain a precision current source to charge an appropriate capacitor. However, forming a high precision resistor on an integrated circuit is difficult and may not be workable in desired applications. That is, the use of an on chip resistor with an on chip current source typically provides very poor precision (approximately 35%) because of the variations in resistor processing in an integrated circuit. That is, the manufacturing processes used to create an on chip resistor are difficult to control to pro□uce precise and consistent results. The result is often poor precision due to process variations in the manufacturing process that can lead to inconsistencies among various integrated devices. In addition, temperature variations impact the on chip resistor and on chip current source to cause operational variations that would have to be compensated for with additional circuitry. Accordingly, it would be desirable to obtain a high precision oscillator in an integrated circuit without additional external components or circuitry for compensation.